1. Field of the Invention
This invention relates to wavefront sensors and more particularly to a wavefront sensor which can operate with either a continuous or pulsed incoming optical beam and which utilizes a modified cyclic interferometer including a polarizer with an optical aperture.
2. Description of the Prior Art
A light beam can be significantly distorted as it passes through the atmosphere or other transmission media. This problem, for example, has limited the resolution of images received by telescopes of stellar bodies deep in space. In addition, atmospheric distortion has posed a severe restriction on attempts to efficiently irradiate objects with laser beams especially when those objects are located great distances from the emitting laser. Other distortions present in practical optical systems also add to beam quality degradation resulting in loss of system performance.
Different wavefront correction systems are used in overcoming such distortions. A critical part of such wavefront correction system is a wavefront sensor. Generally, the main objective of wavefront sensors is to measure the spatial distribution of any wavefront deformations (i.e., wavefront deviations from a given surface such as a flat or spherical surface). Wavefront deformation is expressed as the optical path difference (OPD) in terms of the optical wavelength.
The different types of wavefront sensors may be categorized into two groups--those that measure the OPD distribution directly and those that measure this distribution indirectly. The indirect method involves first measuring the wavefront slope distribution and then from the wavefront slope distribution calculating the OPD distribution. The wavefront slope error distribution is commonly measured by either using a Hartmann sensor approach or one of the different forms of shearing interferometric concepts (i.e., linear or radial shearing). Examples of these approaches are in the following patents:
U.S. Pat. No. 4,141,652 entitled, "Sensor System for Detecting Wavefront Distortion in a Return Beam of Light," issued to J. M. Fineleib;
U.S. Pat. No. 4,518,854 entitled, "Combined Shearing Interferometer and Hartmann Wavefront Sensor," issued to R. A Hutchin;
U.S. Pat. No. 4,575,248 entitled, "Wavefront Sensor Employing Novel D. C. shearing Interferometer," issued to B. A. Horwitz and A. J. MacGovern.
These indirect methods require calculations to convert slope data into OPD data. These calculations can be made in either real time or off-line and with either dedicated hard wired circuitry or in software. Hard wired data processing is designed to respond to a specific task, is expensive and requires bulky equipment. The conversion of data by using different software based schemes tends to be time consuming and/or requires high performance computing equipment.
Direct measurement of the optical path length with respect to a given reference surface (such as a flat) is most often made by a wavefront sensor that is based on one of several different interferometric schemes. There are double path and common path interferometer-based schemes. Examples include Twyman-Greene and Mach-Zehnder configurations.
The accuracy of double path methods, by their nature, relies heavily on the quality of the components. Furthermore, the prior art configurations, whether common or double path, require complicated optical systems and most of them can only be utilized for incident beams which are continuous.
An example of the Twyman-Greene approach can be found in U.S. Pat. No. 4,346,999 entitled, "Digital Heterodyne Wavefront Analyzer," issued to N. A. Massie.
Electronic interferometric techniques, using heterodyne measurement methods such as that described in the Massie '999 patent, or in U.S. Pat. No. 4,188,122 entitled, "Interferometer," issued to S. Holly and N. A. Massie have been recently introduced for facilitating electronic readout of OPD values from optical fringe fields thereby resulting in greatly improved reproducibility and accuracy with high spatial, temporal and OPD resolutions.